1. Field of the Invention
The present invention relates to an image forming process and apparatus using a toner for developing a latent electrostatic image formed on a latent electrostatic image bearing member such as a photoconductor. The process and apparatus are used in the fields of, for example, electrophotography, electrostatic recording and electrostatic printing.
2. Description of the Related Art
In electrophotographic image formation, a latent image is electrostatically formed on an image bearing member such as of a photoconductive substance, and charged toner particles are applied to the latent electrostatic image to thereby form a developed visible image. The visible image formed from the toner is transferred to a transfer medium such as a sheet of paper and is fixed thereon by action of, for example, heat, pressure or solvent evaporation to form an image output.
Processes for charging toner particles to form a visible image in the electrophotographic image formation are roughly classified as “two-component developing processes” in which toner particles are charged as a result of friction by stirring and mixing with carrier particles, and as “one-component developing processes” in which toner particles are charged without the use of carrier particles. The one-component developing processes are then classified as magnetic one-component developing processes and non-magnetic one-component developing processes based on whether or not magnetic power is used for maintaining the toner particles on a development roller.
To fix such a developed visible image, hot-press image-fixing is preferably employed, in which a fixing member such as a fixing roller or fixing belt is directly pressed to an unfixed image to heat and fuse the toner to thereby fix the toner image onto an image bearing member such as a sheet of paper. The hot-press image-fixing technique has a high thermal efficiency, can use a simple image-fixing mechanism and can use a fixing member that can be produced at low cost.
Among such image-fixing mechanisms, an image-fixing mechanism (hereinafter referred to as “belt fixing system”) using a belt heat-transfer medium (hereinafter briefly referred to as “fixing belt”) has been widely used for energy conservation and miniaturization of apparatus.
According to the belt fixing system, the belt can be in contact with the surface of the toner for a longer time in image fixing process, and the image can be fixed at lower temperature. However, it may often invite an “offset” phenomenon in which the fused toner adheres to the surface of the belt, since the fusing of the toner is accelerated. The belt fixing system often invites offset particularly in color toners, since the color toners comprise a binder resin having a sharp molecular weight distribution and exhibiting sharp melt in order to provide sufficient transparency and appropriate gloss for high quality and to fuse the toner more rapidly in image-fixing.
As the belt heat-transfer medium, endless or regular belts comprising a heat-resistant resin are often used, these resinous belts must be improved in mechanical durability.
In addition, an external additive and other components of the toner adhere to the belt heat-transfer medium to thereby wear or damage the belt. Thus hot offset may occur and the belt heat-transfer medium itself may break.
To solve these problems, a variety of proposals have been made on image fixing mechanisms and toners for developing latent electrostatic images.
An example of proposed image-fixing mechanisms can be found in a presentation “Discussion on On-demand Image Fixing Techniques (A-11)” in Japan Hardcopy '94, Technical Conference of The Imaging Society of Japan (Jun. 23-24, 1994).
However, improvement in the image-fixing mechanism alone cannot fundamentally solve the above problems.
To prevent the offset in image fixing, a releasing material such as wax may be added to a toner. However, when the wax has inappropriate properties or is dispersed inappropriately in the toner, the wax leaves or bleeds out from a surface of the toner in long-term usage in a development unit. In a two-component developer, the bled out wax may stain the surface of the carrier to thereby deteriorate charging ability of the toner. In a one-component developer, the wax may fuse and attach to a development roller or a blade for controlling the thickness of the toner to thereby inhibit uniform development of a toner image. From the viewpoint of development, therefore, the amount of a wax should be preferably minimized.
Conventional toners prepared by kneading and pulverization generally have an irregular shape, have a broad particle diameter distribution, exhibit low fluidity and low transfer ability, require high image-fixing energy and have low stability in charge due to uneven charges among toner particles. Such pulverized toners often adhere to a carrier, a latent electrostatic image bearing member such as a photoconductor and/or a blade in image fixing, although the toners exhibit higher releasability. This is because more wax is exposed from a surface of the pulverized toners, since a toner material article tends to break at portions where the wax occurs as a result of pulverization, and such broken portions of the wax constitute surfaces of the toner particles.
As a possible solution to these problems in the pulverized toners, a method for producing a toner by polymerization has been proposed. The method does not require kneading and pulverizing processes and can save energy, shorten the production time, improve yields of products and can significantly reduce cost. As compared with the pulverization method, the polymerization method can easily produce toner particles having a sharp particle diameter distribution, enables the wax to be included in the toner particles easily, can yield toners having significantly improved fluidity and can yield spherical toner particles.
However, toners prepared by the polymerization method still have some problems. The polymerized toners have insufficient mechanical properties, although they have a higher sphericity of toner particles than the pulverized toners, since surface tension acts upon toner particles in polymerization process. The polymerization method cannot easily control the shape of toner particles to a desired shape, although the resulting toner has stable charge ability and excellent transfer ability.
Among such polymerization processes, suspension polymerization is widely employed. However, the suspension polymerization must use, as a monomer for a binder resin, styrene monomers and/or acrylic monomers harmful to the human body and yields a toner containing these harmful components, thus inviting environmental pollution. The polymerized toner includes a wax inside thereof, which wax is hardly exposed from the particle surface, and does not so much contribute to image fixing as compared with the pulverized toner which has a wax component exposed from its surface, although the polymerized toner is resistant to adhere to a photoconductor when used in an actual image-forming apparatus. The polymerized toner is therefore disadvantageous in power consumption. When the polymerized toner contains a larger amount of a wax or contains a wax having a larger dispersed particle diameter and is used as a color toner, it yields a color image with deteriorated transparency and is not suitable for a toner for forming an image through an overhead projector (OHP).
Such a polymerized toner can also be prepared by emulsion polymerization. However, the emulsion polymerization must use styrene monomers as a monomer component. In the emulsion polymerization, unreacted monomer components, an emulsifier, and a dispersing agent may not be significantly completely removed from toner particles, thus inviting environmental pollution due to the toner.
Certain toners are prepared by dissolution suspension. The dissolution suspension can use a polyester resin that can undergo image-fixing at low temperature. However, a high-molecular weight component is used in a process for dissolving or dispersing toner materials such as the resin and a coloring agent in a solvent, and the resulting solution or dispersion has an increased viscosity to thereby decrease the productivity. Japanese Patent Application Laid-Open (JP-A) No. 09-15903 discloses a toner prepared by the dissolution suspension and having a spherical shape with a rough surface for more satisfactory cleaning up of the residual toner. However, when wax is added to a composition of this toner as a releasing agent, the wax is not sufficiently uniformly dispersed and is not exposed from the toner surface, and the coloring agent (pigment) is not sufficiently uniformly dispersed as compared with the pulverized toners. In addition, a polymer binder (binder resin) for use herein cannot be freely designed, since a high polymer may become particles in the solvent to thereby increase the viscosity. Thus, the toner may not exhibit satisfactory releasability.
JP-A No. 11-133665 proposes a dry toner containing an elongation product of a urethane-modified polyester and having a practical sphericity of 0.90 to 1.00 as a toner binder in order to improve the fluidity, image-fixing properties at low temperature, and hot-offset resistance of the toner. JP-A No. 11-149180 and JP-A No. 2000-292981 disclose dry toners having a small average particle diameter, which are excellent in fluidity, transfer ability, storage stability at high temperature, image-fixing properties at low temperature, and hot-offset resistance. In the publications, these toners are prepared by a method including a process for increasing molecular weight in which an isocyanate-containing polyester prepolymer is subjected to additional polymerization with an amine in an aqueous medium.
The toners prepared by the method, however, contain heterogeneously dispersed pigment and wax components and yield images with low transparency and decreased chromaticness (colorfulness). The toners cannot exhibit satisfactory releasability when they are used in an oil-less image fixing procedure, since they do not contain a satisfactorily controlled dispersed particles of releasing agent. When the toners are used for forming a color image on an OHP sheet, the image is dark or dim due to a large particles diameter of the dispersed releasing agent.
Objects and Advantages
Under these circumstances, an object of the present invention is to provide an image forming process and apparatus that can fix images satisfactorily at low temperature for energy saving, exhibit satisfactory hot-offset resistance and storage stability at high temperature, satisfactory gloss for use as color toners and sufficient transparency for use in an OHP and can form images with very stable good quality in actual use over a long period of time.